<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        ul {
            margin: 0;
            padding: 0;
            list-style: none;
            position: relative;
            margin-top: 100px;
        }

        ul li {
            position: absolute;
            border: skyblue 1px solid;
            border-radius: 50%;
            /*  */
            left: 50%;
            top: 50%;
            transform: translate(-50%, -50%);
        }
    </style>
</head>

<body>
    <ul>
        <li style="width: 50px;height: 50px;"></li>
        <li style="width: 100px;height: 100px;"></li>
        <li style="width: 150px;height: 150px;"></li>
        <li style="width: 200px;height: 200px;"></li>
    </ul>
</body>

</html>